An image forming apparatus of a tandem type is conventionally provided, which includes a plurality of image forming units arranged along a sheet conveying belt. The image forming units sequentially transfer toner images of respective colors to the sheet on the belt. In order to ensure image quality, a technique referred to as “registration” has been adopted to such apparatuses. In this technique, positional deviation of images is detected and corrected.
For example, a known image forming apparatus includes a function to detect and correct steady positional deviation due to misalignment of components of image forming units (photosensitive drums, optical members of exposure units, etc.), or the like. The apparatus includes also a function to detect and correct varying positional deviation having one or more cycles due to eccentricities of the photosensitive drums or belt supporting rollers, gear pitch errors of gears for driving these members, or the like. During one of these functions being performed, a pattern having a plurality of marks of respective colors is formed on the belt by the image forming units. The positions of the marks are measured by an optical sensor, and their deviation amounts from respective ideal positions are detected. Then, based on the detection result, the deviation of an image to be formed of each color is corrected.
During the detection of the steady positional deviation being performed, the pattern is conventionally formed on the belt in a state where the varying positional deviation is corrected, and then the pattern is measured.
However, the accuracy in the steady-deviation detection may deteriorate due to errors contained in the detection and/or correction of the varying positional deviation.
Therefore, there is a need for an image forming apparatus that can ensure the accuracy in the steady-deviation detection.